In the manufacture of semiconducting devices, the most commonly used materials are crystals of germanium and silicon in the form of single crystals. These crystals may or may not be modified by the use of dopants which control the conductivity of the germanium or silicon crystal. Thin film semiconductors are also known, such as semicrystalline and amorphous silicon films and molybdenum trioxide films. Dopant materials are often introduced into the semiconductor crystal by forming a plasma of dopant vapor. The dopant ions, which are selectively extracted from the plasma, are accelerated to provide sufficient energy for their implantation. Thin films associated with integrated circuit technology are typically deposited by pyrolysis reactions, hydrolysis, R-F sputtering, evaporation, anodization, chemical vapor deposition, electrodeposition and sintering processes.
Tin, also, has a known semiconducting modification: .alpha.-tin or gray tin. This modification can be commonly obtained in the form of a fine powder. Although rods, foils or wires may be obtained by compressing the gray tin powder into such forms, and single crystal gray tin can be drawn, thin film semiconductors of gray tin are not known.
Typically, glow discharge plasma processes, in which various gases and vapors can be transformed into solid films, yield transparent insulating and conducting films. For example, Hynecek U.S.A. Pat. No. 4,140,814, issued Feb. 20, 1979, describes a process whereby a transparent conductive layer of SnO.sub.2 is deposited on a dielectric substrate by an RF plasma-assisted chemical vapor reaction of carbon dioxide with an organotin compound such as tetramethyl tin. Thin film formation from a plasma can be effected in a glow discharge reactor which can be inductively or capacitatively coupled to an exciting radio frequency power source. Operating parameters, such as reactor geometry, the arrangement of the radio frequency coil or of the electrodes, the composition of the feed materials, flow rate of gaseous matter, system pressure and input power, are effective to influence the composition, uniformity and depositional characteristics of such plasma-deposited thin films. Typically, films obtained by plasma deposition employing organic feed materials are organic solids and insulators; although there are a limited number of known organic semiconductive materials, see Sansman et al. U.S. Pat. No. 4,080,332, issued Mar. 21, 1978 and Griffith et al. U.S. Pat. No. 4,102,873, issued July 25, 1978. It is possible to obtain conductive films by employing organometal and inorganic feed materials and, in some cases, especially at high power inputs, electrically conducting amorphous carbon films can be obtained. Conversely, thin semiconducting films by plasma deposition of organometal feed materials are not known.
In accordance with the present invention, there are provided semiconducting organotin thin films, as well as processes for preparing such films by plasma deposition of organotin feed materials. The invention provides processes for modifying the conductivity of such semiconductor films and, in another aspect, provides devices having conductive elements disposed in a semiconductive matrix and processes for producing such devices.
Briefly, the present invention is based on the discovery that certain organotin compounds in vapor form can be employed as feedstock in a glow discharge plasma deposition process to provide thin films having a metallic appearance. The composition of such films is characterized by a carbon content less than and a tin content higher than the carbon and tin contents, respectively, of the original organotin feed material. The films have an unexpected semiconducting property with a conductivity in the range of 10-1.times.10.sup.-2 OHM.sup.-1 CM.sup.-1. The semiconducting organotin films produced in accordance with the invention characteristically have an amorphous structure. It has also been found that heat treatment of the thin films of this invention at temperatures not exceeding the melting point of tin (T.sub.f =505.degree. K.=232.degree. C.) can effect an increase in conductivity of the films without loss of the semiconducting function. It has also been found that high energy irradiation, such as by electron or particle beams, of organotin semiconductor thin films of the invention results in the conversion of the irradiated area to a conductor, whereby there can be produced devices having conductive elements disposed in a semiconducting matrix.